You say : "The kernel virtual area (3 - 4 GB address space) maps to the first 1 GB of physical RAM. The 3 GB addressable RAM available to each process is mapped to the available physical RAM."

It seems to me that physical addresses are completely sequential on PCs (there are no holes in the physical address space). Thus how can the kernel map the userspace virtual memory to physical RAM above 1 GB even if the machine has less than 1 GB RAM ?

I must admit I have trouble understanding your explanations...